A desert is defined by its lack of moisture, receiving less than 250 millimeters of precipitation annually. These arid regions, which can be hot or cold, are not scattered randomly across the globe. Global atmospheric patterns, driven by the uneven heating of the Earth, initiate massive air movements that circulate heat and moisture. These patterns dictate precisely where vast, dry areas will form.
The Subtropical Desert Belts
The majority of the world’s deserts are concentrated in distinct bands that circle the planet. These arid zones are consistently found near the 30 to 35-degree latitude lines, both north and south of the equator. This geographical area is known as the subtropical high-pressure zone, where atmospheric conditions naturally suppress rainfall. Historically, sailors referred to these calm, dry zones as the “horse latitudes.” This belt represents the main latitude where global circulation mechanics create persistent dryness.
How Atmospheric Circulation Creates Aridity
The underlying reason for these desert bands is the Hadley Cell, a massive system of atmospheric circulation. This cell begins at the equator, where intense solar heating causes warm, moist air to rise high into the atmosphere. As this air rises, it cools, and the moisture condenses to fall back to the surface as heavy, tropical rain, creating a belt of low pressure.
The now-dry air mass then flows poleward in the upper atmosphere, cooling further as it travels away from the equator. Around 30 degrees latitude, this cooled, dense air begins to sink back toward the surface. This descending air mass compresses as it falls, which causes it to warm significantly.
Warm air has a greater capacity to hold moisture than cool air. Therefore, the descending air actively absorbs any surface moisture rather than releasing it as precipitation. This constant subsidence of dry, warming air creates a persistent band of high pressure at the surface, leading directly to the subtropical deserts.
Localized Influences on Desert Formation
While the Hadley Cells establish the main desert latitudes, secondary factors can create or intensify aridity outside of the 30-degree bands. One such influence is the rain shadow effect, where a major mountain range physically blocks moisture-laden air. As air is forced up the windward side, it releases its moisture; the dry air then descends on the leeward side, creating a desert, such as the Patagonian Desert.
Another localized influence involves cold ocean currents flowing along the western coasts of continents. Currents like the Humboldt Current off South America or the Benguela Current off Africa chill the air directly above the water. This cold air inhibits evaporation and stabilizes the atmosphere, preventing the formation of rain clouds. Consequently, coastal deserts like the Atacama and the Namib are extremely arid despite their proximity to a massive body of water.
Prominent Deserts of the Thirty-Degree Zones
The world’s largest deserts perfectly illustrate the global pattern established by the subtropical high-pressure belts. The immense Sahara Desert in North Africa sits squarely within the northern 30-degree zone, extending across the continent. Similarly, the Arabian Desert, which covers much of the Middle East, is a direct result of this same atmospheric mechanism.
In the Southern Hemisphere, the pattern is equally clear with the Kalahari Desert in Southern Africa and the Great Australian Desert, which is a collective term for several arid regions across the interior of the continent.